Substantial technical effort has been directed to the preservation of perishable fluid food products such as milk products, natural fruit juices, and liquid egg products which may contain a wide variety of microorganisms and which are excellent culture media for microorganisms. Practical preservation methods which have found significant commercial application predominantly utilize heat treatment such as pasteurization to inactivate or reduce the microorganism population. For example, milk products are conventionally pasteurized at a minimum temperature of about 72.degree. C. for 15 seconds (or equivalent time/temperature relationships) to destroy pathogenic bacteria and most of the nonpathogenic organisms, with degradative enzyme systems also being partial or totally inactivated. However, products processed in this manner are still generally nonsterile and have limited shelf life, even at refrigeration temperatures and may still contain significant levels of bacteria, bacterial spores, and other microorganisms which are not generally killed, or not completely killed, at the temperatures used. The shelf life of liquid foodstuffs may be substantially extended by higher heat treatment processes such as ultra-high temperature (UHT) pasteurization, which generally employs temperatures from about 94.degree. C. for three seconds to about 150.degree. C. for one second in conjunction with aseptic packaging. Often, destruction of essentially all bacteria and bacterial spores can be obtained at the high temperature range of the UHT pasteurization process. Typically, however, such high temperature treatment adversely affects the flavor of the food product, at least partially denatures proteins contained therein, and/or otherwise adversely affects the desired properties of the fluid food product. Other approaches to liquid food preservation, which also have certain disadvantages, include the use of chemical additives or ionizing radiation.
The bactericidal effects of electric currents have been investigated since the end of the 19th century, with various efforts having been made to utilize electrical currents for treating food products. See, e.g., U.S. Pat. Nos. 918,531, 1,900,509, 2,428,328, 2,428,329, and 4,457,221; and German Patents 1,946,267 and 2,907,887. For example, U.S. Pat. No. 918,531 describes a method of treating milk using relatively low voltage (about 1100 volts) where the milk is cooled during the period of treatment in order to prevent overheating of the milk due to the heating effect of the applied voltage. Generally the lethal effects of low-frequency alternating current with low electric field strength have been largely attributed to the formation of electrolytic chemical products from the application of current through direct contact electrodes, as well as ohmic heating produced by current flow through an electrically resistive medium. As described in U.S. Pat. No. 3,594,115, lethal effects of high voltage arc discharges have also been attributed to electrohydraulic shock waves. However, such electrolytic chemical products may be undesirable in fluid foodstuffs. Thus, the utilization of explosive arc discharges to produce microbiologically lethal shock waves has not found wide-spread application in the provision of edible liquid foodstuffs having extended shelf life.
More recently, separately from the art of food preservation, the effect of strong electric fields on microorganisms in non-nutrient media has been studied as a mechanism for reversibly or irreversibly increasing the permeability of the cell membrane of microorganisms and individual cells. See, e.g., Sale et al., "Effects of High Electric Fields on Microorganisms. III. Lysis of Erythrocytes and Protoplasts," Biochmica et Biophysica Acta, 163, pp. 37-43 (1968); Hulsheger et al., "Killing of Bacteria with Electric Pulses of High Field Strength," Radiat. Environ Biophys, 20, pp. 53-65 (1981); Hulsheger et al., "Lethal Effects of High-Voltage Pulses on E. coli K12," Radiat. Environ. Biophys. 18, pp. 281-288 (1980); Zimmermann et al., "Effects of External Electrical Fields on Cell Membranes, Bioelectrochemistry and Bioenergetics, 3, pp. 58-63 (1976); Zimmermann et al., "Electric Field-Induced Cell-to-Cell Fusion," J. Membrane Biol., 67, pp. 165-182 (1982); Hulsheger et al., "Electric Field Effects on Bacteria and Yeast Cells," Radiat. Environ. Biophys; 22, pp. 149-162 (1983); Zimmermann et al., "The Development of Drug Carrier Systems: Electrical Field Induced Effects in Cell Membranes," Biochemistry and Bioenergetics, 7, pp. 553-574 (1980); Jacob et al., "Microbiological Implications of Electric Field Effects. II. Inactivation of Yeast Cells and Repair of Their Cell Envelope," Zeitschrift fur Allgemeine Mikrobiologic, 21, 3, pp. 225-233 (1981); Kinositas, Jr., "Formation and Resealing of Pores of Controlled Sizes in Human Erythrocyte Membrane," Nature, 268, 4, pp. 438-440 (August, 1977); Neamann et al., "Gene Transfer into Mouse Lyoma Cells by Electroporation in High Electric Fields," IRI Press Limited, Oxford, England, pp. 841-845. The application of high electric fields to reversibly increase the permeability of cells has been used to carry out cell fusion of living cells and to introduce normally excluded components into living cells. Electric fields in non-nutrient media have a direct lethal effect upon microorganisms with the rate of kill dependent upon the field strength above a critical field level and the duration of the applied high voltage pulse or pulses.
These studies postulate the cell membrane as the site of a critical effect, that is, the reversible or irreversible loss of membrane function as the semipermeable barrier between the cell and its environment. An external field of short duration is assumed to induce an imposed trans-membrane potential above a critical electric field value, which may produce a dramatic increase of membrane permeability. Because an increase in cell permeability prevents the counteracting of differences in osmality of the cell content and surrounding media, exchange or loss of cell contents, or cell lysis, irreversible destruction may occur as secondary mechanisms in non-nutrient media which limit the ability of cells to repair themselves, and which adversely affect permeable cells through osmotic pressure differences between the medium and the interior of the cell.
Even more recently, U.S. Pat. Nos. 4,695,472, 4,838,154, 5,048,404, and 5,235,905 have provided methods and apparatuses for providing fluid food products having extended shelf life using electric field treatment. These methods and apparatuses generally allow the temperature of the fluid to be treated to increase due to ohmic heating effects during application of the electric field. Such increases in temperature can result in reduced quality of the fluid treated. Such deleterious effects on the quality of the fluid can include changes in flavor, color, appearance, aroma, functionality of proteins, and the like. Although, in many cases, such changes may not significantly effect the use of the treated fluid, they can still have a significant affect on the ultimate consumer's acceptance of the food product. In these prior art methods, the temperature was prevented from reaching excessively high levels by either decreasing the electric field strength or decreasing the duration of treatment which will, of course, decrease the efficiency of microorganism kill. Generally, it has been found that these processes, while providing excellent levels of general microorganism kill, often do not provide the levels of bacteria, bacterial spore, and/or fugal spore kill that would generally be desired. Moreover, it has generally been found that high temperature processes, such as, for example, UHT pasteurization, often provide good levels of bacteria and bacterial spore kill but with significant and adverse changes in the flavor, appearance, odor, and/or function of the product. Thus, it would be desirable to provide improved methods whereby foodstuffs can be treated with pulsed electric fields, especially for longer periods of time and at higher electric field strengths, whereby bacteria and bacterial spore levels can be significantly reduced or eliminated while minimizing the detrimental effects caused by elevated process temperatures.
Accordingly, it is an object of the present invention to provide methods for extending the shelf life of perishable pumpable food products such as dairy products, natural fruit juices, fluid egg products, beer, wine, soups, stews, gravies, particulate food suspensions or slurries, and other pumpable food products while minimizing the exposure of the food product to excessive temperatures at which significant and undesirable changes tend to occur in the flavor, appearance, odor, or function of the food products. Another object of the present invention is to provide a process for preparing a pumpable foodstuff with significantly reduced levels of microorganisms, said process comprising:
(1) preheating a pumpable foodstuff to a temperature in the range of about 30.degree. to 120.degree. C.;
(2) passing the pumpable foodstuff of step (1) through a plurality of treatment zones arranged in series, wherein the pumpable foodstuff is passed through each treatment zone in turn, wherein the pumpable foodstuff in each treatment zone is subjected to multiple pulses of an electric field having a field strength of at least 10 kV/cm;
(3) cooling the pumpable foodstuff after passage through each treatment zone in step (2), except the last treatment zone, so that the temperature of the pumpable foodstuff entering the next treatment zone is in the range of 30.degree. to 120.degree. C.; and
(4) rapidly cooling the pumpable foodstuff from the last treatment zone in step (2) to a storage temperature;
wherein the temperature of the pumpable foodstuff in each treatment zone in step (2) is maintained below a predetermined temperature and wherein the total duration of electric field treatment and the temperature of the pumpable foodstuff in the treatment zones are sufficient to provide a foodstuff with significantly reduced microbial levels with minimal changes in the flavor, appearance, odor, or function.
Still another object of the present invention is to provide a process for preserving a pumpable foodstuff, said process comprising:
(1) passing a pumpable foodstuff through a plurality of treatment zones arranged in series, wherein the pumpable foodstuff is passed through each treatment zone in turn and wherein the pumpable foodstuff is subjected to multiple pulses of electric fields having field strengths of at least 10 kV/cm in the plurality of treatment zones;
(2) cooling the pumpable foodstuff after passage through each treatment zone in step (1), except the last treatment zone, before it enters into the next treatment zone; and
(3) rapidly cooling the pumpable foodstuff from the last treatment zone in step (1) to a storage temperature;
wherein the temperature of the pumpable foodstuff in each treatment zone in step (2) is maintained below a predetermined temperature and wherein the total duration of electric field treatment and the temperature of the pumpable foodstuff in the plurality of treatment zones are sufficient to preserve the pumpable foodstuff while minimizing changes in the flavor, appearance, odor, or function of the pumpable foodstuff.
These and other objects of the present invention will become apparent from the following detailed description and the accompanying drawings.